Some high intensity discharge lamps, mercury lamps for example, can start without the use of a starter if the open circuit voltage available from a ballast is relatively high. Other high intensity discharge lamps, such as high pressure sodium lamps, have employed simple pulse type starters for years. Normally, the existing starters operate from a 60-cycle voltage available from a ballast and primarily function to break down the gas within the discharge lamp.
Newer high intensity discharge lamps, metal halide lamps in particular, contain large amounts of free iodine which has a natural affinity for electrons. This iodine quickly absorbs the energy contained within a narrow voltage pulse such as provided by the better known forms of starters available. Thus, in order to insure adequate breakdown of such discharge lamps, it becomes necessary to increase either the peak pulse voltage, the peak pulse voltage width or the peak pulse voltage repetition frequency if the necessary increased energy is to become available. The peak voltage that a starter is allowed to generate depends on the lamp socket and circuit wiring. This limit is typically 4000 volts. Since there are readily available starters with peak pulse voltages rated up to 4000-volts which fail to start the above-mentioned newer types of high intensity discharge lamps, it was determined that increases in the peak pulse voltage width and/or the peak pulse voltage repetition rate were necessary if starting of the newer type lamps was to be effected.
Once having achieved breakdown of the high intensity discharge lamp, high levels of voltage and current are required to continue conductivity of the lamp for the period required to effect a shift from a glow condition to an arc condition. Thus, the provision of the above-mentioned relatively wide peak pulse voltages having relatively high pulse repetition rates becomes necessary to insure the desired starting of the lamp. Moreover, inadequate starting energy can undesirably leave a lamp in a glow state whereupon rapid electrode erosion results or the lamp will undesirably flash and go out which is also deleterious to electrode life. Thus high levels of energy are necessary to maintain lamp operation.
Other conditions which may occur due to the lack of a sufficient supply of energy include lamps which may light and conduct on only one-half cycle due to cathode imbalance. Thereupon, the ballast tends to saturate and supply the lamp with currents as high as 10 to 20 times the rated lamp current for many cycles which obviously can be damaging to the electrodes.
Also, it is desirable for a starter to function with either a lead ballast, such as a capacitor and small inductor or a lag ballast such as an inductor. With a lead or primarily capacitive type ballast, a parallel injection type starter tends to provide energy which is absorbed by the power line. Accordingly, reference is made to U.S. Pat. No. 3,753,037 issued to Kaneda. In contrast, a series injection starter does not see the power line as a load and therefore does not inject energy back into the power line.